1. Field of the Invention
The present invention relates to a lifting apparatus. More particularly, the present invention relates to a lifting apparatus which can maintain the top of a load platform at a constant height regardless of the change in load or temperature. Most particularly, the present apparatus relates to a lifting device which utilizes a glycol fluid to maintain a load platform at a specific height. Since the load platform is maintained at a specific height in a particularly stable condition by the use of glycol, with hydraulic circuitry and components combined with compressed air as an energy source, the invention is very useful for workers who need to be located at the optimum position for the job they are performing.
2. Description of the Prior Art
There are a number of known load platforms in the art which will maintain people or objects in a sitting or standing position for various specific reasons.
An example of such a device is that disclosed in U.S. Pat. No. 4,319,666 to Hunter. Hunter discloses a service jack to be mounted on a vehicle lift track for movement into desired places along the runways of the lift rack. The service jack includes a carriage suspended between the runways, an elongated vehicle lifting jack assembly operateably supported by the carriage for vertical movement toward and away from the member supported on the runways, a fluid pressure lifting member between the carriage and the vehicle's lifting jack assembly, releasable safety device responsive to raising of the vehicle lifting jack assembly for preventing accidental reverse movement thereof, and devices for guiding and maintaining the vehicle jack assembly in a substantially level attitude in all positions.
U.S. Pat. No. 4,688,760, entitled Lifting Apparatus, issued to Garman et al., shows a lifting apparatus including a base, a platform disposed above the base, a thrust device therebetween to lift the platform relative to the base, and a platform stabilizer enclosed within the thrust device to connect the platform and base in parallel. The platform stabilizer includes an upper extensible linkage device having upper end portions connected to the platform, a lower extensible linkage device having lower end portions connected to the base, and a connecting arrangement connecting lower end portions of the upper extensible linkage device to upper end portions of the lower extensible linkage device so that the linkage devices are constrained to move in unison. The thrust means is preferably pneumatically inflatable to apply lifting forces directly to the platform independently of the platform stabilizing means.
U.S. Pat. No. 5,299,906, entitled Self Adjusting Pneumatic Load Elevator, issued to Robert M. Stone shows a self adjusting load elevator having a base, a horizontal load platform, a vertically expandable scissors linkage coupled between the load platform and the base, and an air actuator chamber. The air actuator chamber is formed of a compressible bellows and a fixed volume reservoir. The bellows is compressible between specified maximum and minimum bellows heights which correspondingly determine the substantially different maximum and minimum bellows volumes, and coupled between scissored linkage and the load platform. The air reservoir is coupled to the bellows and has a fixed volume that is substantial compared to the difference between the maximum and minimum bellows volumes. A normally closed air inlet/outlet valve is coupled to the air actuator.
The three examples listed above are all pneumatically operated. It has been found that pneumatic operation is not satisfactory for a worker standing on a work platform because of the "springingness" of the platform. Further, pneumatically only operated devices are subject to some effect because of wide temperature swings, and may need to be constantly adjusted.
Hydraulically operated lifting platforms are also known and could be satisfactory, but hydraulic fluid is not viewed as environmentally acceptable in work areas, and require a source of energy, normally electrical, to operate.
Therefore those skilled in the art of lifting devices continue to search for a satisfactory solution of how to provide a stable platform for a worker which would not have springy feel, not be affected by temperature variations, and utilize low pressure compressed air as an energy source.